Boron nitride (BN), by analogy to carbon, can appear in a cubic crystalline structure (cBN) similar to diamond and a hexagonal crystalline form (hBN) like that of graphite. BN can also exist in lonsdaleite-like wurtzitic (wBN), rhombohedral (rBN) and turbostratic (tBN) phases. Among these allotropes sp3-bonded cBN in zinc-blend structure is the most attractive crystalline form. The strong covalent bonds and high atom density make cBN a unique material with properties comparable to those of diamond. cBN has the widest band gap (>6 eV) among all the covalent-bonded materials, the second highest hardness next to diamond, extraordinary chemical inertness, high electrical resistivity, high carrier mobility, and extremely high thermal conductivity. Cubic BN has the second highest elastic modulus and second highest thermal conductivity next to diamond, but it surpasses diamond in chemical properties. Cubic BN has considerably higher oxidation temperature, higher graphitization temperature and high chemical inertness. While diamond is dissolved and graphitized in iron and other ferrous materials, cBN does not react even with molten ferrous materials.
The unusual combination of its physical and chemical properties makes cBN a potentially very useful material for the fabrication of cutting tools, thermal, optical, and high-temperature and high-frequency electronic devices. Although cBN has a slightly lower hardness and smaller thermal conductivity than diamond, it is superior to diamond in mechanical applications due to its chemical inertness to ferrous materials up to temperatures of 1500-1600 K. Therefore, cBN is a very good material for machining ferrous materials and their composites.
Furthermore, cBN can be doped for either p-type or n-type conductivities while n-type doping of diamond is still problematic. Hence, cBN is also a better candidate for fabricating high-speed, high-power electronic devices operating at high temperature [O. Mishima, J. Tanaka, S. Yamaoka, and O. Fukunaga, Science vol. 238, pps. 181-183, “High-temperature cubic boron nitride p-n junction diode made at high pressure”, 1987].
The study of the synthesis and characterization of cBN and diamond started almost simultaneously in the early 1960s [R. H. Wentorf, Jr., J. Chem. Phys. Vol. 36. pps. 1990-1991, “Preparation of semiconducting cubic boron nitride”, 1962]. To date, cBN has been commercially synthesized in large quantities as powders with sizes ranging from submicron to millimeters by a high-pressure high-temperature (HPHT) method [N. V. Novikov, Diamond Relat. Mater., vol. 8, pps. 1427-1432, “New trends in high-pressure synthesis of diamond”, 1999]. Together with HPHT diamond, HPHT cBN has a large market especially in cutting tools and wear parts.
The severe nature of the HPHT methods and the limited size of cBN grains produced, however, rule out some of its attractive potential applications. Research in chemical vapor deposition (CVD) of diamond films has progressed well and CVD diamond has been implemented in some industrial areas such as protective coats, cutting inserts, heat sinks, optical windows, etc. Methods have been developed for growing diamond films at reduced temperatures and pressures. However, diamond films are not well-suited for cutting or abrading ferrous materials. Diamond reacts with iron forming soots at the high temperatures generated during cutting operations.
Accordingly, it would be desirable to provide a method for the implementation of cBN films on tools. However, because known cBN films grow via soft amorphous and turbostratic boron nitride (aBN and tBN) and exhibit high stress, they suffer from poor adhesion. That is, the known cBN films have an aBN/tBN/cBN interfacial structure that has a number of defects, particularly dangling boron bonds, which cause the films to be undesirably highly reactive and also cause high instability. These films tend to delaminate from the substrate to which they are adhered. For this reason, these cBN films have not been implemented in practice.
Cubic boron nitride films can be deposited by both physical vapor deposition (PVD) and CVD methods. For PVD methods, bombardment of substrates by energetic species (tens to hundreds of eV) is known to be important to the formation of the cBN phase [T. Yoshida, Diamond Relat. Mat., vol. 5, pps. 501-507, “Vapour phase deposition of cubic boron nitride”, 1996; P. B. Mirkarimi, K. F. McCarty, and D. L. Medlin, Mater. Sci. Eng., vol. R21, pps. 47-100, “Review of advances in cubic boron nitride film synthesis”, 1997]. Since ion bombardment is associated with a considerable internal stress that restricts the maximal thickness of adherent, non-delaminating cBN films to ˜200 nm, this presents a further obstacle to its practical application.
For CVD, by using boron trifluoride as the boron source, thick cBN films may be deposited by DC jet plasma CVD and microwave plasma CVD. It is known that fluorine can selectively etch non-cubic BN. The substrate bias needed for the formation of cBN phase is dramatically reduced using CVD and correspondingly the residual stress in the films was reduced. However, the cBN films deposited by this method still grow via the layered structure of amorphous and textured hexagonal (turbostratic, tBN) phases [W. J. Zhang, S. Matsumoto, K. Kurashima, and Y. Bando, Diamond Relat. Mater., vol. 10, pps. 1881-1885, “Structure analysis of cBN films prepared by DC jet plasma CVD from an Ar—N2—BF3—H2 gas system”, 2001], serving as incubation layers for cBN nucleation. However, the existence of the amorphous/turbostratic interface causes random orientation of the cBN film, and diminishes the film adhesion in humid environments. Therefore, adherent cBN films to a substrate without aBN/tBN transition layers, non-sensitive to humidity and even grown in an epitaxial relationship to the substrate are highly desired to meet practical applications.